A simple transistor metal detector on beats. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple transistor metal detector with beats. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

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Schematic diagram

The proposed design (Fig. 2.1) is one of the many types of BFO (Beat Frequency Oscillator) metal detectors, that is, it is a device based on the principle of analyzing beats that occur when two signals of close frequencies are mixed.

Rice. 2.1. Schematic diagram of a simple transistor metal detector

The design of the device uses two simple LC-oscillators, made on transistors T1 and T3. The operating frequency of these generators is determined by the parameters of the circuits included in the collector circuits of the corresponding transistors. The circuit of the first oscillator, which is the reference, is formed by a capacitor C1 with a capacity of 330 pF and a coil L1. In the circuit of the second, measuring, generator, a variable capacitor C7 with a maximum capacitance of 300 pF and a search coil L2 are used.

The outputs of the generators through resistors R1, R5 and capacitor C4 are connected to the base of the transistor T2, which amplifies the beat frequency signal. From the collector of the transistor T2, the amplified signal is fed to the headphones BF1. The volume level of this signal is controlled by a variable resistor R6.

Since the operating frequencies of the generators are in the medium wave range, their signals are not audible in phones. When it is possible to achieve precise tuning of each generator to the same frequency, there will also be no sound signal in the phones. If, with the help of capacitor C7, the measuring oscillator is tuned to almost the same frequency as the reference oscillator, then the beat frequency signal will be heard in the phones. If there are no metal objects in the area of action of the search coil L2, the operating frequency of the measuring generator remains unchanged, so the beat frequency will also be unchanged. In this mode, possible frequency deviations can only be due to the unstable operation of both generators.

When a metal object appears in the coverage area of \u2b\u2bthe search coil L7, the resonant frequency of the LXNUMXCXNUMX circuit will change. As a result, the operating frequency of the measuring generator and, as a result, the frequency of the beat signal will change. It is these changes that serve as a source of information about the detection of a metal object.

A simple transistor metal detector is powered from a source B1 with a voltage of 9 V.

Details and construction

All parts of a simple transistor metal detector, with the exception of the search coil L2, capacitor C7, resistor R6, connectors X1 and X2 and switch S1, are located on a printed circuit board 100x50 mm in size (Fig. 2.2), made of one-sided foil getinax or textolite.

There are no special requirements for the parts used in this device. Naturally, it is recommended to use any small-sized capacitors and resistors that can be placed on a printed circuit board without any problems.

Coil L1 of the reference generator is wound with PEL wire with a diameter of 0,1-0,2 mm on a core with a diameter of 8 mm and contains 100 turns. Coil L1 can be wound on a ferrite core or on a paper tube without a core. Capacitor C7 can be any air-dielectric variable capacitor with a maximum capacitance of about 300 pF, such as a tuning capacitor from any old radio.

Simple transistor metal detector

Rice. 2.2. The printed circuit board (a) and the location of the elements (b) of a simple transistor metal detector

The printed circuit board with the elements located on it and the power supply are placed in any suitable plastic or wooden case. A tuning capacitor C7, a volume control R6, a switch S1, as well as connectors X1 and X2 for connecting the search coil L2 and headphones BF1, respectively, are installed on the housing cover.

For the manufacture of the L2 search coil (Fig. 2.3), you will need to cut a circle with a diameter of 100 mm from plywood or other material (getinax, textolite) with a thickness of 1,5-2,5 mm. The circle should be divided into sectors with an angle of 40 ° and in these places cuts should be made to the center at a distance of 20 mm from the edge. It is necessary to pass a wire with a diameter of 0,2-0,3 mm (for example, the PEL brand) into the slot and wind 30 turns around the turn. A convenient handle can be attached to the search coil made in this way. Coil L2 is connected to the printed circuit board through a small connector.

Simple transistor metal detector
Rice. 2.3. Search coil design L2

As a power source V1, you can use, for example, a Krona battery or two 3336L batteries connected in series.

Establishment

When using serviceable parts, a properly assembled metal detector starts working almost immediately. Adjustment of the device should be carried out in conditions when metal objects are removed from the search coil L2 at a distance of at least 1,5 m.

Since this section deals with a simple design, when you first turn on the phones, you will hear the signals of different harmonics. Therefore, when setting up the device, you should choose the strongest signal using the capacitor C7 and adjust its volume with the R6 knob. The choice of the strongest harmonic can be done by ear or using an oscilloscope or frequency meter.

This completes the process of setting up a simple transistor metal detector.

Operating procedure

In practical use of the metal detector in question, capacitor C7 should be additionally adjusted to the signal of the strongest harmonic and adjust its volume using regulator R6.

If now any metal object is in the area of action of the search coil L2, then the pitch in the telephones will change. When approaching some metals, the frequency of the beat signal will increase, and when approaching others, it will decrease. By changing the tone of the beat signal, having a certain experience, one can easily determine what metal, magnetic or non-magnetic, the detected object is made of.

Author: Adamenko M.V.

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